Concerning the recent discussion about molecular clocks -
Another point to consider is that the products of homologous genes in different
species may acquire additional functions without gene duplication.
This has occurred for several enzymes which have been recruited as bulk
structural components of the lens in different vertebrate lineages. The first
example of this was LDH-B which in many birds and crocodiles also serves as
epsilon-crystallin (Wistow et al, Nature, 1987, 326, 622) and may account
for almost half the total protein in the lens (Wistow etc, PNAS, 1990,87,6277).
There is a single gene for "both" proteins (Hendriks etc, 1988, PNAS, 85,7114).
With two interesting exceptions, those species which use e-crystallin have
acquired specific amino-acid sequence changes in LDH-B, presumably selected
for the lens structural role of the protein.
Allan Wilson (TIBS, 1987, 3, 241) noted that this probably explained his
observations of a "markedly uneven rate" of change in LDH-B in birds, made
over 20 years earlier.
graeme at mge2.nei.nih.gov